Method and apparatus for signaling employing polarized lights



April 8,

METHOD AND APPARATUS FOR SIGNALING EMPLOYING POLARIZED LIGHTS FiledMarch 27, 1946 P. :4. LEE

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April 8, 1952 ms'mou AND APPARATUS FOR SIGNALING EMPLOYING POLARIZEDLIGHTS 5 Sheets-Sheet 3 Filed March 27, 1946 INVHVTOR. ayl ai/ e/ an a:

METHOD AND APPARATUS FOR SIGNALING EMPLOYING POLARIZED LIGHTS FiledMarch 27, 1946 P. H. LEE

April 8, 1952 5 Sheets-Sheet 4 )gl INLELTOR. BY 9 P. H. LEE

April 8,1952

METHOD AND APPARATUS FOR SIGNALING EMPLOYING POLARIZED LIGHTS w m m A am w M z A? z N m f m Patented Apr. 8, 1952 PATENT OFFICE METHOD ANDAPPARATUS FOR SIGNALING EMPLOYING POLARIZED LIGHTS Paul H. Lee,Cambridge, Mass., assignor, by mesne assignments, to the United Statesof America, as represented by the Secretary of the Navy ApplicationMarch 27, 1946, Serial No. 657,577

2 Claims.

This invention relates to the art of signaling and more specifically hasreference to method and apparatus for transmitting signals, information,orders, and the like in the form of substan-- tially instantaneous lightflashes which are re- 5 ceived and converted into electrical impulsesused to effect the actuation of an electrical utilization circuit.

Objects of the invention are to provide apparatus for transmittingsignals in the form of light flashes of high intensity and substantiallyinstantaneous duration; to provide for the transmission of signals bymeans of light flashes emanating from one or more flash lamps of thegaseous discharge type and to provide means for predeterminedly firingor discharging such lamps; to provide one or more channels ofcommunication for each flash lamp used in a signaling system bypredeterminedly modifying the light emitted by each flash lamp throughthe use of light filter means which may be selectively positionable inthe path of the light emitted by the flash lamp and which may compriseat least one light fllter having the ability to predeterminedly polarizelight; and to provide optical means in a signaling system whereinsignals ar transmitted by light flashes whereby to direct light from aflash in a predetermined direction.

Other objects of the invention are to provide signal receiving apparatushaving one or'more receiving channels each formed by an individual lightresponsive means, such as a photoelectric cell, which converts receivedlight energy into electrical energy for delivery to an electricalutilization circuit; to provide a signal receiver having a plurality oflight responsive means, each capable of converting changes of lightenergy into changes of electrical energy; to provide means forrestricting the exposure of one or more light responsive devices in asignal receiver to light of predetermined characteristics by means oflight filters which may comprise at least one fllter having the abilityto predeterminedly polarize light: to provide, in a signal receiver ofthe character described, means for amplfying changes of electricalenergy produced by any light responsive means thereto for effecting theactuationof a utilization circuit and interpreting means for controllingdelivery of the amplified changes of electric energy to such utilizationcircuit.

Further objects of the invention are to provide a signaling systemhaving a transmitter for eifeoting the transmission of signals by lightflashes of high intensity and substantially instantaneous duration incombination with a receiver spaced therefrom and having light responsivemeans receiving the light flashes and converting them into electricsignals adapted to actuate a utilization circuit, to provide atransmitter in a signaling system of the character described whichtransmits signals by predeterminedly fired gaseous discharge means suchas one or more flash lamps, the light flash from any one or which lampsmay be directed in one or more channels of communication formed by lightfilter means which may include a light polarizer and through which theflash is transmitted to a receiver comprising means for converting eachflash into an electric signal employed to actuate an electricalutilization circuit; and to provide a signaling system having incombination a transmitter which transmits signals by light flashesdirected through light filter means to produce signals in the form offlashes of light of predetermined characteristics and a receiver spacedfrom the transmitter and having one or more receiving channels eachformed by an individual light responsive means. such as a photoelectriccell, which is capable of converting changes of light energy to changesof electrical energy, together with light filter means for restrictingthe exposure of each light responsive means to a flash of light ofpredetermined characteristics which corresponds to the characteristicsof light transmitted by the transmitter and, in conjunction therewith,means for amplifying the electric signals produced by the lightresponsive means for eflecting the actuation of an electricalutilization circuit and interpreting means controlling the delivery ofthe amplified signals to the electrical utilization circuit.

Still further objects of the invention are to provide a method ofsignaling wherein light flashes or substantially instantaneous durationand high intensity are predeterminedly created and transmitted in apredetermined direction and are received at a distance from theirorigin, converted into signals in the form of electrical pulses whichare amplified and interpreted for controlled delivery to a utilizationcircuit; to provide a method of signaling comprising predetermlnedlygenerating light flashes of substantially instantaneous duration and ofhigh intensity. transmitting light emitted by such flashes in a desireddirection while predeterminedly modifying the same, whereby to transmitlight signals in selected channels of communication. and receiving thelight flashes transmitted in each channel of communication in anindividual receiving channel and converting received light signal intoelectrical energy for effecting the actuation of an electricalutilization circuit; and in a method of the character described, toprovide for coding the light signals by varying their physicalcharacteristics. such as their shape, polarization, intensity or timeperiod of duration. as well as for codin by varying the time between thegeneration of light flashes, and also in such a method to provide forreceiving light flashes ofdiflerent light characteristics, such aswavelength and/or plarization, and converting the received light flashesinto signals in the form of electrical energy in accordance withwavelengths and/or polarization of the light signals, amplifying theelectrical energy pulses and interpreting the energy pulses inaccordance with their shape, intensity, and time period of durationwhereby to control the delivery of the pulses to an electricalutilization circuit.

Other objects of the present invention are to provide a signaling systemor the type described above having improved security and jam-freecharacteristics wherein light flashes of substantially instantaneousduration are used for signaling and means are provided forpredeterminedly changing a characteristic of the light flashes, such astheir polarization, so as to make the light flashes unintelligible to anunauthorized receiver; and to provide a receiver having meanssynchronized with corresponding means in a transmitter which will enableit to receive light flashes predeterminedly altered so as to utilizesaid flashes in a utilization circuit; and to pro- 'vide a transmitterwhich can predeterminedly emit meaningless flashes of light of acharacter which will not affect an authorized receiver but will affectan unauthorized receiver whereby a garbling of the message will beaccomplished.

Another object of the present invention is to provide a transmitter foremitting substantially instantaneous discrete flashes of light. acharacteristic of which is modulated in accordance with audio signals,whereby an audio frequency channel of communication is provided; and toincrease the security or an audio frequency transmitter using modulateddiscrete pulses of light by providing means in said transmitter forfurther modulating said flashes of light in a predetermined manner.

With these and other objects in view, the in vention comprises theapparatus possessing the construction. combination of elements andarrangement of parts and the method comprising the several steps and therelation of one or more of such steps with respect to each of the otherswhich are exemplified in the construction and method hereinafterdescribed. and the scope of the application of which will be indicatedin the claims.

The above and other objects and novel features of this invention willmore fully appear from the following detailed description when the sameis read in connection with accompanying drawings. It is to be expresslyunderstood, however. that the drawings are for the purposes ofillustration only and are not intended as a deflnition of the limits ofthe invention. reference being primarily had for this latter purpose tothe appended claims.

For a fuller understanding of this invention, reference should be had tothe accompanying drawings, in which:

Figure l is a schematic block diagram of a 4 transmitter embodying onefeature of the present invention;

Fig. la is a modification of the invention shown in Fig. 1;

Fig. 2 is a schematic block diagram of a receiver embodying one featureor the present invention;

Fi 2a is a, modification of the invention shown in Fig. 2;

Fig. 3 is a schematic block diagram or another modification of areceiver embodying the present invention;

Fig. 4 is a schematic block diagram of a transmitter adapted to transmitaudio-frequency modulated signals in accordance with the presentinvention:

Fig. 5 is a schematic block diagram of a further modification of atransmitter which may be used in the present invention;

Fig. 6 is a schematic detailed circuit diagram of a transmitter adaptedto transmit signals for the control of a guided missile; and

*Flgs. 7 and 8 are detailed circuit diagrams showing a receiver adaptedto receive signals, interpret these signals, and apply them to autilizatiiim circuit such as the control of a guided miss e.

Light flashes such as those generated on the discharge or flring of aflash lamp or tube of a gaseous discharge type used in the presentinvention have an intensity which is considerably greater than sunlight.Flashes produced by such lamps are substantially instantaneous and mayhave a time period of duration of the order of from /2o,eoo to/i,ooo.ooo of a second. The unique characteristics of flashes producedby such gaseous discharge devices are advantageous in signaling in thatthe instantaneous duration of the flashes permits a sharp cutofl betweensuccessive signals with consequently easy diflferentiation therebetweenwhile the high intensity of the flashes permits transmission of signalsfor relatively long space distances even under adverse lightingconditions that is to say, in strong sunlight.

In providing a signaling system wherein signals, iniormation, orders andthe like are transmitted by light flashes of high intensity and ofsubstantially instantaneous duration to a receiver, use is made of atransmitter which directs light emitted on the discharge of one or moregaseous-discharge devices to a receiver which is spaced from thetransmitter and which converts the light flashes received thereby toelectrical pulses adapted to be selectively delivered to a utilizationcircuit. A signaling stem of such character is adapted for use withtransmitting and receiving stations which may be stationary or movable.Such systems are particularly useful in eflecting the direction of amoving object which carries a receiving station. In such instances, themoving object may be directed from a moving or a stationary transmittingstation. In addition, systems of this type are useful in transmittingcode or message-bearing signals from a moving or stationary transmittingstationto a moving or stationary receiving station. They may also beused in transmitting audio signals, such as speech.

The principles involved in providing a signaling system of the characterforming the subject 1 matter of the invention are schematicallyillustrated in the block diagrams which form Figs. 1 through 5. Briefly.these principls involve the use of an optical switch for directinglight.

of specific electrical circuits, several specific circuits beinghereinafter described in detail.

Turning now to Fig. 1. there are shown two flash lamps or tubes no andll of the type described in the patent to H. E. Edgerton, 2,351,603,June 20. 1944, and the patent to Germeshausen. 2,277,698, March 31,1942. Since this type of flash lamp is well known in the prior art, adetailed description thereof is not considered necessary. In general,however, this flash lamp is a very high-resistance, gaseous dischargetube, which, when ionized by an induction current, breaks down to anextremely low resistance, thereby allowing the passage of an extremelylarge current therethrough, and thus permitting practicallyinstantaneous discharge of a high voltage condenser therethrough. Apreferred type of tube is that produced by the General E1ectric Companyunder the designation Fl' 19.

To control these flash lamps or tubes, there are provided firingcircuits l2 and I3. These circuits, which are similar to those shown inthe Edgerton patent mentioned above, comprise means for applying highvoltages to the flash tubes and also means for ionizing the flash tubesto cause discharge of the high voltage therethrough. As can be seen fromFig. 1, firing circuit l2 controls flash tube I0, while firing circuiti3 controls flash tube II. There is also provided a selector circuit Mwhich will trigger either the firing circuit [2 or the firing circuitIS, in accordance with a predetermined signal from a control circuit 15.The control circuit may be of any suitable type, such as a standardtelegraph key, teletypewriter, or a miniature control stick of anaircraft, adapted to close a number of predeterminedly spaced switches.Numerous other types of control circuits are contemplated by the presentinvention and will be apparent to those skilled in the art.

There is also a suitable power supply l6 which will give suitableenergization to the various cir-'- cuits and tubes employed in thepresent invention. For guiding the emission of light from the flash tubeI0, there is shown a flanged reflector H, a lens 18, and a. polarizingfilter Is. This is a preferred system of communication, since the lightis concentrated in a predetermined path. The polarizing filter i9 ispreferably an infrared filter and may be either a plane polarizing orcircular polarizing filter. A similar optical system is provided for theflash tube I I, wherein the reflector H, the lens l8 and an infraredpolarizing filter l9 are provided. In this case, it is preferred thatthe polarizing filter l9 be of an opposite sense from the polarizingfilter Iii-in other words, one may be a plane polarizing fllter and theother may be a circular polarizing filter, or both may be planepolarizing filters, one being adapted to polarize in a vertical plane,while the other is adapted to polarize in a horizontal plane. It is alsopossible, and in some cases desirable, for one filter to be infraredwhile the other filter is ultraviolet. The reason for preferring to useinfrared or ultraviolet fllters is that the light passed by such filtersis invisible to the eye.

It is apparent that the specific type of optical system shown is subjectto many variations. For

instance, light may be emitted through 360 by the use of a proper lenssystem, and it is equally possible to practice the present invention bythe elimination of all optical systems and filters. It is preferred,however, to have light of distinct characteristics emitted from each ofthe two flash lamps, so that light reaching a receiver from one flashlamp may be distinguished by the receiver from light reaching thatreceiver from the other flash lamp. There will thus be formed twoseparate channels of communication.

It is also possible to practice the present invention by the use of justone flash tube. This feature of the invention will be discussed in moredetail in connection with the discussion of the receiver circuit shownin Fig. 3.

Turning now to a discussion of Fig. 2, there is shown a receiver adaptedto operate with the transmitter of Fig. 1. As shown in Fig. 2, there areprovided two photosensitive devices 30 and 3|. These devices may beeither of the photoemissive type, the photovoltaic type, or thephotoconductive type. In a preferred modification of the invention, thephotosensitive devices are of the photoemissive type and are infraredsensitive photocells made by the Farnsworth Radio and TelevisionCompany. The output of these two photocells is fed into a differentialamplifier 32, which will amplify a difference in the signals generatedby the two photocells. If the signals are the same, however, there willbe no amplification. The output of the difierential amplifier is fed toan automatic volume control 33 and also an interpreting circuit 34. Theautomatic volume control may be of the standard type and is used foradjusting the level of amplification to the point where shot noise andother noise generated within the photocells and the amplifier circuitwill not operate the interpreting circuit. This automatic volume controlautomatically compensates between the allowable amplification level forday. and night use, it being apparent that there will be much morerandom noise generated in the photocells during bright daylight use-thanwould be generated at night during the darkness.

The interpreting circuit 34 is adapted to interpret the signals fed toit by the differential amplifler. This circuit may be responsive to theinrepetition I e signals, the pulse length of the sigtensity of thesignal quency o nals or any other variation normally used for 17 variouscode transmissions. It will, of course, bei obvious that theinterpreting circuit must be designed for the particular type of signalssent out by the transmitter. In one modification of the presentinvention, it is contemplated to have the interpreting circuit operateon signal spacing constituting dots and dashes of the Morse code. Inanother modification of the invention, the signals fed to theinterpreting circuit are spaced in such a way as to be changed by theinterpreting circuit to signals for use with a teletypewriter. Inanother modification of the invention, the signals fed to theinterpreting circuit are for the use of controlling the flight of aguided missile.

The utilization circuit 35 may be of any kind, such as a loud speaker orhead phones for use with audio signals or Morse code. It may be theaileron and rudder controls of a guided missile or it might be ateletypewrlter.

The photocell 30 has an optical system associated therewith comprising areflector 31. a lens 38 and a polarizing filter 38; and the photocell 3!has a similar reflector 31', lens 38' and polarizing filter 39'. Asuitable power supply 3 is provided for energizing the various circuits.

In order to understand the operation of Figs. 1 and 2, it is preferableto consider their opera tion together with one simplified type ofsignal. If in Fig. 1, it is desired to transmit a code composed of twocharacters such as the dot and dash of'the Morse code or the mark andspace pulses of a teletypewriter; the fiash tube filter 'Il may be avertically polarizing infrared filter,

and the filter I9 may be horizontally polarizing infrared filter. Anormally open switch having two pairs of independently actuated opposedcontacts-may be employed-to control the selector circuit I 4 to energizeeither the firing circuit l! or the firing circuit 13, depending uponwhich pair of contacts of the switch is closed. For the purposes ofillustration, it will be assumed that actuation of the firing circuit [2corresponds to a dot in Morse code, while the actuation of the firingcircuit l3 corresponds to a dash. When the switch is operated in adirection to close the "dot" contacts. the firing circuit I2 isactuated. This fires the fiash tube II which emits from the polarizedfilter I! a vertically polarized infrared fiash of light. The filter 39associated with the photocell III in Fig. 2 may be a verticallypolarizing infrared filter and the filter 39' associated with thephotocell 3i may be a horizontally polarizing infrared filter.- When thevertically polarized infrared flash of light reaches the receiver, ithas no ellect'upon the photocell 3| since it is blocked by thehorizontally polarizing filter 39'. It does. however, pass through thefilter 39 and reach the photocell 80. This causes a very definiteunbalance between the outputs of the two photocells and the differentialamplifier therefore amplifies the si'gn'alreceived and generated by thephotocell Ill. Since this signal is in the form of a very sharp pulse,the automatic volume control has no effect upon the gain of theamplifier during the time that this signal is being amplified. and thesignal is greatly amplified and fed to the interpreting circuit 34.Assume that the amplifier feeds a negative pulse to the interpretingcircuit 34 and that the interpreting circuit is designed to produce adot when it is energized by a negative signal. This dot is then fed tothe utilization circuit 35. which in this case may be a light bulb or apair of head phones. The dot is then reproduced by either audio-orvisual means.

Returning now to Fig. 1, assume that the normally open switch previouslymentioned is placed in the dash position. This energizes the selectorcircuit I in" such a way that it triggers the firing circuit IS. Thetriggering of the firing circuit It causes the fiash tube II to fire anda pulse ofhorizontally polarized infrared light is transmitted throughthe filter It. When this fiash .of light reaches the receiver of Fig.2', it is rejected by the polarizing filter 39 but passesthrough thepolarizin filter SI and causes an energization of photocell 3|. Thisunbalances the circuits so that the difi'erential amplifier produces apositive pulse. As in the case of the negative pulse discussed above,the automatic volume control has no efiect upon the particular signalbeing amplified, and this amplified positive pulse is fed to theinterpreting circuit 34. The interpreting circuit, which is designed togenerate a dash upon the receipt of a positive pulse. so generates adash, which is ted to the earphones in the utilization circuit 35.

It should be noted in connection with Fig. 2 that by "the arrangement01' the circuit shown therein, there is produced a receiver which isquite insensitive to the influence of light coming from sources otherthan either of the flash tubes of Fig. 1. This is due to two features ofthe receiver circuit: The first is that the output of the two photocells30 and'3l is fed into a difierential amplifier which amplifies only thediilerence between these two outputs. Light striking these twophotocells in the same intensity does not allow the amplifier to operateand pass a signal to the interpreting circuit. Thus, if a bright flashof sunlight, for instance, hits the photocells ll and 3 I, it energizesboth phoiocells equally, since sunlight is polarized in all directionsand is passed in the same amounts by the two filters 3! and 39'. Thesame applies to any other unpolarized lightstriking the two photocells.The second feature of the circuit which prevents its response to otherthan light signals from the fiash tubes is the automatic volume control.As pointed out previously, this automatic volume control has a shortenough time delay so that it will prevent the amplification of lightwhich changes, in intensity comparatively rapidly. A time delay of /50of a second is sufiiciently short for most purposes. Since the timededay of /50 of a second has no efiect upon a light dash of a totalduration of the order of between 500 and l microseconds. a light fiashfrom the fiash tubes is not affected by the automatic volume control andthe difierential amplifier is allowed to amplify these short pulses. Thedifferit is felt that the means for accomplishing the desired resultwill be readily apparent from a discussion oi the more complicatedcircuits which will be described in detail later.

In Fig. in there is shown a somewhat modified optical system, whereinfiash tubes II and H are employed as in Fig. 1. These fiash tubes havelenses. II and I8 and polarizing filters I! and I9. It should be noted,however, that the filters l9 and I! are mounted upon a rotary disc 23.driven by a shaft 22 from a motor 2|. Thus, the polarization and type oflight emitted from fiash tube 10, for instance, is continuously changed.At one flash, it might be infrared light vertically polarized, and atthe next fiash it might be ultra violet light circularly polarized.There is also shown an additional fiash tube 20. having a lens l8" and afilter is" similar to lens II and filter I 9. The flash tube 20 iscontrolled by a firing circuit 24 which is, in turn, controlled by asynchronizing circuit 25. The fiash tube 20 can be used to synchronize amotor ll driving a rotary disc 40 shown in Fig. 2a, in which case thesynchronizing circuit 25 controls the firing of the fiash tube 20 so asto permit synchronization between the two motors. As will be readilyunderstood by those skilled in the art. motor 2i may be a constant speedmotor which controls the synchronizing circuit, or the synchronizingcircuit may control the speed oi the motor.

It is also possible to utilize thefiash tube 20 as a garbling flashtube. in order to make the interception of messages more dimcult. Ifthis be the ma-emu.

case, it may be desirable to have the flash tube 20 emit onlyunpolarized light, in which case the filter I!" can be only an infraredfilter and does not polarize the light in any predetermined manner, butallows unpolarized light to pass therethrough. If the flash tube 20 isused only for garbling, it may be fired independently of the operationof the synchronous motor in either a random or predetermined manner. Itis also apparent that the garbling flash tube 20 can be used with thetransmitter circuit of Fig. l where no rotary filters are used.

Fig. 2a shows a receiving circuit adapted to be used with a transmittercircuit of the type described above in connection with Fig. 10. Asshown, there has been added to the circuit of Fig. 2 a rotating disc 40,driven by a motor 4| through a shaft 42. There is a synchronizingcircuit 43 and a photocell 44. This photocell 44 is adapted to have itsoutput fed to the synchronizlng circuit 43, which, in turn, controls thespeed of rotationof motor 4| and also its angular position at anyinstant of time, so as to keep motor 4| in synchronism with the motor 2|of Fig. 1a. The rotating disc 40 carries polarizing filters 39 and 39"which are adapted to be placed in front of the respective photocells asthe disc rotates. The filter 39" is a polarizing filter of the same kindas the polarizing filter l9".

Turning now to a discussion of the operation Y of Fig. 1a and Fig. 2a,it can be seen that there are certain advantages in the optical systemas shown. If, at any one instant, the flash tube 10 is fired. itproduces a flash of light corresponding to the type of filter l9positioned in front thereof at the time of the flash. Since a filter ofthe same type is in front of the corresponding photocell 30 in thereceiver, the firing of the flash tube l energizes the photocell 30 inthe receiver. The same is true of the firing of the flash tube ll, sincethe filter in front thereof at any instant of time, corresponds to thefilter in front of the photocell 3|. Since the transmission of lightbetween the transmitter and the receiver is of a negligible time, nospecial precautions are necessary in the synchronization of the rotatingfilters carried on the disc 23. Whenever the flash tube 20 is fired, itgenerates a pulse of light having a color and polarization dependingupon the type of filter l9" used. If this flash of light is used tosynchronize the receiver motor 40, it may be advantageous to have thefilter I9" polarized in a manner unlike any of the filters I! or l9carried by the rotating disc 23.

It is advantageous to use the circular disc 23 carrying the filters l9and 9' in some cases. since it makes the system less susceptible tojamming by the enemy if the system is used in war time. since it isapparent that continually changing types of polarization used would beextremely difficult to duplicate so as to effectually hindercommunications. For instance, the disc 23 may carry four infraredfilters polarized difierently, four green filters polarized differently,and four blue filters polarized differently. Anyone attempting to jamthe system would have to send a signal of the proper color, and of theproper polarization and at the proper time to energize flash tubes 30 or3|. With the system mentioned above, there are 12 combinations and, atany instant of time, only two of the 12 are effective to energize eitherof the photocells in the receiver. It is, of course, apparent thatevenmore combinations may be employed which would make jamming even moredifiicult. This is particularly 10 true where more photocells areemployed and the circuit is designed so that a specified two of thephotocells must be energized at any instant in order to operate theinterpreting circuit.

It is also possible, as explained above, to utilize the flash tube 20for the purpose of garbling the signals. In this case, it might bedesirable to have the filter l9" nonpolarizing. If this be the case, itwould be extremely difficult for anyone not authorized to receive thesignals transmitted by flash tubes In and II to distinguish from thefiashes produced by tubes I0, I and 20.

It is equally possible to have the individual filters rotating aroundtheir opticalaxes as well as around the axis of the disc carrying thefilters, thus giving the filters a planetary motion.

In order to use one photocell in the transmitter and receiver, insteadof two, the circuits of Figs. 1 and 2 can be slightly modified. As ameans of doing this. the firing circuit l3 and the flash tube I l areomitted from the transmitter of Fig. 1 and the receiver is modified asshown in Fig. 3.

In Fig. 3 there is provided a photocell 50 preferably of thephotoemissive type, an amplifier 52 which can be a high-gain amplifier,and an automatic volume control 54. The automatic volume control ispreferably adjusted to prevent the amplifier from amplifying any noisegenerated either in the photocell or the amplifier circuit itself. andit has a sufficiently fast time circuit so as to prevent a signal frompassing through the amplifier upon comparatively rapid changes in theintensity of the light striking photocell 50, such as would be caused bychanges in theintensity of light on a partially cloudy day or when thereceiver was swung from a position away from the sun to a positionpointing generally towards the sun. The automatic volume control.however, has a sumciently slow response so that it does not prevent theamplification of a substantially instantaneous signal such as isgenerated by the flash tube In of Fig. 1, although it may lower theaverage peak voltage of a following signal. The output of amplifier 52is fed into an interpreting circuit 56 which may be a standard countercircuit or other decoding circuit. As an example, it may have such anarrangement of parts that one single pulse spaced in time from any otherpulses will operate a certain tube to close a relay, while a number ofpulses spaced close together energize the tube to maintain the relayclosed for the whole group of closely spaced pulses. Since this generaltype of circuit is well known in the art, a detailed discussion thereofwill not be included here. It is only necessary for the relay to have atime lag for opening greater than the spacing between the pulsesconstituting a dash signal. The output of the interpreting circuit 56 isfed to the utilization circuit 58 which may be earphones or ateletypewriter or any other suitable apparatus for utilizing electricalenergy. It is obvious that many types of interpreting circuits can beutilized, some operating upon the pulse length, others upon the pulseintensity, or others on the pulse shape. For the purpose of explainingthe present invention, however, the interpreting circuit will beconsidered as operating upon the delay-relay principle.

One of the simplest modifications of a Morse code transmitter is onewhere the selector circuit H of Fig. 1 triggers the firing circuit I! atrapid intervals when energized by the control circuit. The flashinginterval of the firing circuit I! may be made approximately equal to thelength of a dot in Morse code, and such a firing circuit may beconveniently controlled by means of an automatic telegraph key of thetype well-known in theart. An automatic telegraph key comprises ahorizontal operating handle pivoted about a vertical axis and operatingtwo sets of normally open contacts. The first set of contacts consistsof a fixed contact and a cooperating oscillatory contact carried on aweighted resilient arm which acts as a pendulum when the operatinghandle is moved to a dot position and causes the contacts associatedtherewith to close for a short interval during each oscillation of theresilient arm. thus producing a series of dots. The second set ofcontacts are connected in parallel with the first set and consist of afixed contact and a movable contact carried by the operating handlewhich mates with the fixed contact when the operating handle is moved tothe dash" position. Thus the automatic key automatically sends a seriesof dots when the handle is moved in one direction and a continuous dashwhen the handle is moved in the other direction. The semi-automatictelegraph key in the present application is operated in opposite tonormal fashion, since the normal dot" contacts are employed to transmita dash and the normal dash" contacts are used to transmit a dot. Thus,the flash tube l0, for a dot. emits only one flash, while for a dash itemits a series of flashes spaced very close together. when the photocell50 receives a single flash, the amplifier 52 feeds a single pulse to theinterpreting circuit. If a dash is received by the photocell 50, theamplifier 52 feeds a number of pulses to the interpreting circuit 55,and the delay relay stays closed for a longer time, thus reproducingdots or dashes in the utilization circuit 50.

In the above discussion of Figs. 1 and 2, the discussion of theinvention has been limited to the use of two flash tubes and twophotocells for forming separate channels of communication. It is, ofcourse, apparent that more photocells and more flash tubes can be usedto increase the number of channels, and combinations of various types oflight can be utilized for different purposes. For instance, four flashtubes can be used in the transmitter with four different polarizingfilters, and four photocells can be used with four corresponding filtersinthe receiver. These filters can all be of one color and one of themvertically polarizing, another horizontally polarizing, a thirdcircularly polarizing clockwise, and the .fourth circularly polarizingcounterclockwise. It is possible to add four more flash tubes and fourmore photocells polarizing as above, but

having a color diiferent than that used in the first set of polarizers.The receiving circuit can also be made so that it is selective to eitherone photocell being energized or various pairs of photocells beingenergized. Thus, the channels of communication can be expanded almostindefinitely. It is not felt necessary to describe in detail all of thevarious circuits necessary to utilize and effectuate such expandedcommunication channels, since it is believed that the detaileddiscussions of the circuits used with the present invention will makethe modification thereof apparent to those sklled in the art.

, It is also possible to increase the freedom from jamming by making theinterpreting circuit sensitive only to signals having predeterminedshapes, spacing, amplitude,'puise repetition frequency. length, or otherphysical characteristics All such modifications are within the scope of12 the present application and would be readily apparent to one skilledin the art.

Fig. 4 is a schematic diagram of another modiflcation of the inventionwhich contemplates the use of the present invention for transmittingaudio signals and where discrete light pulses are utilized as amodulated carrier. As shown in Fig. 4 there is provided a flash tube 10of the type previously discussed above. There is also shown a firingcircuit 12, a selector circuit H, and a control circuit 16, similar tothose described in connection with Figs. 1 and 2 above. There is alsoprovided a modulator ll adapted to control a light valve 84. A powersupply 80 is utilized for energizing the various circuits mentionedabove. As can be seen in Fig. 4, there is provided a lens 80, adapted tofocus the light emanating from the flash tube I0 through the light valve84, then through a polarizing filter 06. This filter 36 can be any ofthe types discussed above. In order to control the modulator, there isprovided an audiofrequency responsive device such as a microphone 82.

The light valve 84 may be a mechanical shutter or mirror which willcontrol the amount of l ght passing therethrough. A preferred form oflight valve, however, is one which will change the polarization of thelight passing therethrough so that the shifting polarization will changethe amount of light which is passed by the polarizing filter 86, and ofthe devices utilized for changing the polarization a Kerr call is apreferred type due to the fact that it may be operatedwithout anyproblems of mechanical inertia.

In the operation of the circuit of Fig. 4, the control circuit 16 willenergize the selector circuit H which may be, for instance, a 20,000cycle per second oscillator. when energized, the selector circuit feeds20,000 triggering pulses per second to the firing circuit 12. Thiscauses the flash tube 10 to fire 20,000 times per second, and 20,000light flashes of a predetermined intensity reach the light valve everysecond. These 20,000 flashes per second constitute a sort of carrierfrequency which can be amplitude-modulated by the operation of the lightvalve. This amplitude modulation is controlled by the modulator, which,

in turn, is actuated by means such as the microphone 02. As audiosignals reach the microphone 82 they are converted into electricalsignals and are fed into the modulator II; where they operate themodulator to control the light valve 84. Thus, at any instant of time,the instantaneous intensity of a light flash passing through the lightvalve 04 corresponds to the instantaneous intensity of an audio signalreaching the microphone is. There is consequently produced a series ofdiscrete flashes of light. the intensity of each flash varying inaccordance with an amplitude-modulated envelope, corresponding to theaudio signals reaching the microphone 02. In order to receive andinterpret these signals, a circuit of the type shown in Fig. 3 can beused. In this case, the light filter H of Fig. 3 is made to correspondwith the light filter 06 of Fig. 4.

It is also contemplated to modify the circuit of Fig. 4 so that thelight transmitted therefromwill consist of a number of pulses ofsubstantially the same intensity. The advantage of this system is thatit is most difiicult for a person not authorized to receive these lightsignals to interceptthem and interpret them. This is accom plish'ed bymaking the filter 08 of Hg. 4 nonpolarizing or by omitting it entirely.It is preferred, however. to have the filter 80 an infrared filter. Inthis case. all of the other elements of the circuit are the same and itcan be seen that the light valve 84 which is preferably a Kerr cellshifts the polarization of the light passing therethrough in accordancewith the intensity or the audio signals reaching the microphone 82.Since this shift in polarization does not affect the intensity of thelight signals, they may all appear to be of the same intensity. Whensignals of this type, i. e., of the shifting polarization, reach areceiver oi. the type shown in Fig. 3, they are blocked or passed by thepolarized filter 64 of Fig. 3, in accordance with their polarization.Thus. the polarizing filter 04 of the receiver acts as the second haltoi the light valve. The light striking the photocell 50 of Fig. 3 variesin accordance with the amplitude variations of the audio frequencyreaching the microphone 82 in Fig. 4.

In order to improve even further the security of these light signalsfrom interception, it is possible to mount the Kerr cell 84 of Fig. 4 sothat it may be continuously or intermittently rotated. Thus. it it iscontinuously rotated, the plane of polarization thereof is likewisecontinuously rotated and the modulating rotation is superimposed uponthis rotation. If the filter 86 in Fig. 4 is a polarizing filter, it ispreferred to rotate this filter in synchronism with the rotation of theKerr cell 84. If the filter 88 is not a polarizing filter, it is, orcourse, not necessary to rotate it. In order to receive messagestransmitted by a system or the type shown in Fig. 4, where the Kerr cellis rotated continuously and the modulation of the signals isaccomplished by further rotation of the plane of polarization, it isnecessary to have the polarizing filter S4 of Fig. 3 rotate insynchronism therewith. Any suitable means can be utilized forsynchronizing rotation of the filter 64 of Fig. 3 with the rotation ofthe Kerr cell 84 in Fig. 4. Such a means is shown schematically in Figs.la and 20 above.

It is apparent that the relationship between the plane of polarizationof the Kerr cell and the necessary polarizing filter, that is eitherfilter 88 of Fig. 4 or filter 64 of Fig. 3, can be of either of twotypes. The normal plane of polarization of the Kerr cell may be at rightangles to the plane of polarization of the filter or it can be parallelthereto. In the first case, when no signal is applied to the Kerr cell,the filter passes a minimum of light. When a maximum signal is appliedto the Kerr cell, the plane of polarization of light transmittedtherethrough is rotated a maximum and the filter passes a maximum amountof light. In the second situation. when no signal is applied to the Kerrcell, the plane of polarization is such that the filter passes a maximumof light. When the amplitude of the signal applied to the Kerr cell is amaximum. the filter passes a minimum of light.

As mentioned above, it is preferred to use Kerr cell for changing thepolarization of the light passing therethrough. It is possible, however,to use a polarizing sheet of the type known by the trade name Polaroid"mounted in a movable frame. In this case. the polarizing axis of thepolarized sheet is rotated in accordance with the amplitude of themodulating signals. applied to a rotating means such as a solenoid. Asstated above, however, the problems of mechanical inertia make the useor a Kerr cell preferable to this last-described means.

In Fig. 5 there is shown a schematic diagram of another modification ofthe present invention.

14 There is provided a flash tube or the type described above, which isconnected to a switch 88 by a resistance BI and an inductance 92connected in parallel. Between the switch 98 and the switch 01 there arethree condensers 94, 05. and 98, connected in parallel. Connected to theswitch 91 there is a firing circuit 08. Energizing the firing circuit,there is provided a selector circuit 99 which is controlled by thecontrol circuit I00. A suitable power supply I04 is provided for thevarious circuits mentioned above. There is also included a filter MI anda lens I02, which may be of the type discussed in connection with theprevious figures. As can be seen. the selector circuit is connected toswitches 93 and 01 so as to control the operation thereof. In the use orthe transmitter described in Fig. 5, the control circuit I00 energizesthe selector circuit 90 in a predetermined manner and the selectorcircuit controls, in turn, the switches 93 and 91 and also the firingcircuit 94. The condensers 94, 95 and 95, each having a diflerentcapacity produce a different type of flash when discharged through theflash tube 90, and, depending upon their characteristics may give a verybrilliant flash of short duration, a brilliant flash of long duration ora less brilliant flash 01' long or short duration. Since the resistanceand inductance 9i and 92. respectively, are in series with the flashtube and condenser. they also act to modify the flash produced by theflash tube, either changing its slope. duration or intensity. Theselector circuit 90 thus selects any one of the three condensers andeither the resistance or inductance for firing the flash tube. and also.of course, controls the time at which the firing circuit 98 fires theflash tube. It or course. apparent that many other combinations may beemployed for controlling the flash oi the flash tube and will beapparent to those skilled in the art. For instance, with certain typesof signals it might be desirable to use all three condensers in parallelfor flashing the tube.

It is equally apparent that the switches 03 and 91 can be eithermechanical or electronic switches depending upon the particular resultswhich it is desired to accomplish.

Turning now to a description of a detailed embodiment of the invention,as shown in Fig. 6 there is provided a transmitter capable of producingflashes of extremely high intensity light 0! very short duration whichmay be utilized in the various signaling systems described previously.As shown in Fig. 6 there are provided two flash tubes TVTI and TVT2.These tubes are preferably oi the type known as Edgerton flash lamps"and are quite adequately described in the above-mentioned U. S. Patents.As can be seen, each flash tube comprises a coiled, gaseous dischargespace filled with rare gas. This discharge space is indicated as I05. AtI01 there is provided a loop for ionizing the gas in the discharge coilI05. This coil is capacitatively coupled to the gaseous dischargechamber. In parallel with each gaseous discharge tube is a series ofcondensers and resistors for supplying high voltage to the tubes. Inparallel with TVTI are bleeder resistors TRII and TRII. Alsoin. parallelwith this tube are charging condensers TC4 and T05. A minus 4,000-voltsupply is connected at P5 across the condensers and-resistors so thatthe total charge on condenser TC4 and TC! may build up to minus 4,000volts. When the gas in the helical coil I06 of tube TVTI is ionized, thecondenser TC4 and TC! will discharge therethrough, causing a brilliantflash of light to be created. The flash tube TVTI has a similar set ofbleeder resistors TRIS and TR. and similar charging condensers T06 andT01. It also has a minus 4,000-volt power supply at P5.

For flring each of these tubes there is provided a separate firingcircuit. Tube TVTl is utilized for firing flash tube TVTI, while tubeTVTI is utilized for flashing tube TVTZ. Tubes TVT3 and TVTl are of thecold cathode type employing multiple internal grid control and arecapable of conducting very large currents on a light duty cycle. Suchtubes are described in Patents Nos. 2,185,189 and 2,201,167 to Edgerton,Germeshausen and Grier, and are well-known to those skilled in the art.Power supply PI turnishes approximately 375 volts to the plate circuitsoi TVT3 and TVTl. This 375 volts is suitably divided by resistances TRI,'I'R'2, TR3, TRI, TRE. and TRIS, so as to provide suitable voltages tothe various grids of TVTI. Condensers T02 and TCI are also included. Thecircuit of tube TVTI is identical with that Just discussed in connection with tube TVT3, and the various resistors TRI', T32, etc.correspond to their equivalent resistors in the circuit of TVT3.

In the plate circuit of tube TVTI there is provided a condenser TCIwhich is adapted to be charged while tube TVT: is blocked. When tubeTVTi is passing current, it discharges condenser 'ICI. thus passing asurge of current through one halt of autotransformer TI. This currentflow will induce a high voltage in transformer TI, which is applied tothe loop I01 in flash tube TVT2. A similar condenser TCI is provided inplate circuit of flash tu'be TVTl, and it similarly, upon discharging,induces a high voltage in transformer TI connected thereto, this highvoltage having a corresponding efl'ect on the loop II" in flash tubeTVTI. It thus can be seen that whenever tube TVTS is made conducting.the discharge of the condenser TCI produces a high voltage surge in theloop of flash tube TVT2. This voltage surge ionizes the gas in flashtube TVTI and enables this tube to discharge the voltage on condensersT06 and TC! therethrough. Since the resistance 01 the gas in the flashtube TVTI drops almost immediately to an extremely low value, upon beingionized, the discharge of the two condensers is practicallyinstantaneous and an extremely bright flash or light is produced havinga substantially instantaneous duration.

In order that tubes TVT3 and TVTl may be made to conduct atpredetermined intervals and at predetermined sequences of operation, anumber of switches are provided. As can be seen, there are fourswitches, SI. 82. S3 and S4, corresponding respectively to a miniaturecontrol stick of an airplane. This is an illustrative modification andit is obvious that numerous other combinations of switches could beutilized. Connected with the switches are lines 0. D, E, F, and G, theflrst three being connected to contacts III, Ill, and H2, operativelyassociated with commutator H0. The other two lines. F and G. areconnected to tube TVT3 and tube ml respectively. commutator H0 is drivenby a motor M through a 30 to 1 reduction, so as to provide (or thecommutator a speed of approximately 90 R. P. M. The commutator earries ashorting bar III. adapted to engage contacts III, III, and ill insequence. it acting to reduce the potential of those contacts to groundwhenever it engages those contacts. Assuming that switch SI correspondsto an "up signal." S2

to a "right signal," S3 to a "down signal, and S4 to a "left signal,"the operation of the circuit will be described: I! an "up signal" isgiven, the switch SI is closed. When the shorting bar III engagescontact II2, line E is grounded. Since switch I is closed, line G isalso reduced to ground potential, and the potential on the grid of tubeTVT is lowered sufliciently with respect to the other grid thereof, sothat the potential difference existing between the two causes anionization of the gas in the tube and permits the tube to conduct. Thecondenser TCI discharges through the tube TVTl producing a voltage surgeby means of transformer TI' in loop III! of flash tube TVTI, thus firingthe flash tube TVTI. Similarly, switch S3 causes flash tube TV'I'Z tooperate wheneverthe shorting bar engages contact III. Switch 32 causesflash tube TVTI to tire and shortly thereafter it causes flash tube TVT2to flre. Switch S4 causes TVT2 to flre and shortly thereafter causesflash tube TVTI to. fire. If an up,. right signal" is given, switches SIand S2 are both closed and they cause the production of an "up" flashand the two "right" flashes.

In order that one skilled in the art may more readily construct theapparatus described in connection with Fig. 6 above, the various valuesfor the tubes, resistors and capacitors used are enumerated below.

Tubes TVTI FI19 General Electric TVTI BT19 General Electric TVT! 631-P1General Radio TVTI 631-11 General Radio Resistance:

TRI 20 k. ohms 1 watt TRI 20 1:. ohms 1 watt TR! 250 1:. ohms V5 wattTR! 250 k. ohms V watt TR! '75 1:. ohms /2 watt TRJ' 75 k. ohms watt TR25 k. ohms 5: watt TBA 25 k.ohms A watt TRS 250 k. ohms f: watt TRS 250k. ohms '7'; watt TR. k. ohms it watt TRIS 100 k. ohms watt TRII, II,II. It 5megohms2watts Condensers TCI 2 mid. 600 v. TCI 2mid.600v. TC2.007 mfd. 600 v. TCI .007 mid. 600 v. TC3 .01 mid. 600 v. T03 .01 mid.600 v. TC, 5. i, I 25 mid. Cornell Dubilier KGT-6250-1 In order that thelight signals of the two flash tubes may be difl'erentiated there may beas-- sociated with each tube a polarizing fllter. Thus, one tube willtransmit a substantially instantaneous flash oi horizontally polarizedlight. while the other tube will transmit a substantially instantaneousflash oi vertically polarized light. It is also preferable to includewith these polarizers, fllters which will pass'only invisible infraredlight, thus increasing the security of the signals transmitted. Suchflltcrs are preferably of the type described in the oopendingapplication oi Blout and Amon. Serial No. 591,574 flled May 2, 1945,Patent No. 2,444,492, granted July 17 6, 1948, for Optical Filter andMethod of Preparing Same.

A receiver adapted to be used for signals transmitted by a transmitteror the type described in connection with Fig. 6 is shown in Figs. 7 and8. In Fig. 7, none of the optical elements are shown, since it isbelieved that there is schematic representation in the previous drawingssumcient for one skilled in the art to reproduce such a receiver. InFig. 7, there are shown schematically, two light-sensitive circuits. 9.difierential amplifier circuit and an automatic volume control circuit.There are provided two photosensitive tubes VTI and VT2 which arepreferably very sensitive to infrared radiation. In circuit with VTI isa load resistor R1, and in circuit with VT2 is a load resistor R1.Condensers C3 and C4 couple the outputs of the tubes VT1 and VT2respectively to the control grids of tubes VT3 and VT4 oi thedifferential pre-amplifier circuit. R5, in conjunction with C1, servesas a decoupling filter for the voltage supply of VTI and VT2. The gridat VT3 is biased by resistors R2, R3 and R4, while the grid of VT4 isbiased by resistors R2, R3 and R5. In the plate circuit of VT3 isprovided a load resistor R3 and in the plate circuit of VT4 there isprovided a load resistor R1. In the plate voltage supply of both VT3 andVT4 there are resistors R3 and R11, in circuit with the power supply of180 volts. Decoupling the plate voltage supply of VT3 and VT4, there isa condenser C5. Coupling the output of VT3 to the input grid of VT5there is provide a coupling condenser C6. A similar coupling condenserCl is provided between VT4 and VTB. The grid of VT5 is biased byresistors R10, R12 and R13 while the grid of VTS is biased by resistorsR11, R12 and R13. In the plate circuit of VT5 there is provided a loadresistor R15, while in the plate circuit of VT5, there is included aload resistor R14. Decoupling the plate voltage supply of these last twotubes is a condenser C8 and resistor R11. The screen grids of VT3, VT4.VT5 and VTG are all connected together and are provided with a resistorR18 between their common connection and the power supply. Loading theplate circuits of VT5 and VTS are condensers C2 and C2 respectively.

For coupling the output of VT5 with VTI there is provided a couplingcondenser C9, while for coupling the output or VT8 to VT8 there isincluded coupling condenser C 10. Resistances R20, R21, and R23 areutilized for biasing the grid of VT? and resistances R20, R21 and R22bias the grid of VT8. In the plate circuit of VTI there is a loadresistance R18, and in the plate circuit of VT8 there is a loadresistance R15. The screen grids of VTI and VT8 are connected together,and are decoupled from their voltage supply by condenser C13 andresistor R215. The last tube in the amplifier circuit is, a doubletriode VTS. The output of VT'I is coupled to one grid of VT! by thecoupling condenser C12 and resistor R24. The grid of this half of thetube VTS is biased by resistors R25, R28. and R30. The output of VTB isfed to the other half or VT3 by coupling condenser C11 and resistanceR24. The grid of this half of. tube VTS is biased by resistors R25, R21and R30. In the plate circuit of the first half of VTS, there isprovided a resistor R29, while in the plate circuit of the other half ofVT3, there is prc-. vided a resistor R29. The first half of VT! iscathode coupled to the lead 200, while the second half of VT! is cathodecoupled to the 18 lead 201. The lead 200 is also connected by couplingcondenser 016 to one cathode oi double diode VT10. Lead 201 is coupledthrough condenser C15 to the other cathode of double diode VT10.Resistors R31 and R31 are provided in the cathode circuits of VT10.

VT10 is balanced by the capacitance between two short, twisted insulatedwires indicated by C11 which capacity balance the diode. The commonplate connection or VT10 is connected to ground through resistance R32and the A. C. output thereof is fed to VT11 by coupling condenser C22.The grid of VT11 is biased by resistances R33, R31, R30 and R30, and abypassing condenser C25 is provided for R31 and R39. It should also benoted that the cathode circuit of VT10 is variably tapped intoresistance R38. The screen grid of VT11 is biased by resistance R34 andis by-passed to ground by condenser C21. The plate and screen circuitsof VT11 are decoupled from the power supply by resistor R35 andcondenser C20. R35 is a plate load-resistor for tube VT11. R34 and C21are additional filtering elements in the screen circuit of VT11. Theoutput of VT11 is coupled by condenser C20 to the plate circuit ofdouble diode VT12. This tube is shunted by two resistors R40 and R40 inseries and the common junction of these two resistors is connected to apower supply source of minus 4.5 volts. The cathode of VT12 is connectedby resistor R41 directly to the control grid of VT13. Rapid fluctuationsin voltage at this grid are smoothed out by condenser C19, R41 and C13constituting a time circuit. The screen grid of VT13 is by-passed toground by the condenser C18 and is directly connected to a power supplyof 67.5 volts; The plate or VT13 is connected directly to the screengrids of VT3, VT4, VT5 and VTB.

In the operation of circuit described in Fig. '7, when a substantiallyinstantaneous change in the intensity of the light reaching VTI occurs,a signal is generated across resistance R1. This signal is impressed, bymeans of coupling condenser C3, on the control grid of tube VT3. Due tothe arrangement in the common cathode circuit of VT3 and VT4, there is apush-pull amplification of the signal leaving the plates of VT3 andV'T4. Similar action occurs in the other stages of the amplifier. and anamplified voltage is produced across resistances R30 and R28 in thecathode circuit of VTQ. thus producing a signal in lead 200. At the sametime, an amplified signal of opposite sign is produced across R30, and asignal of sign opposite to the signal in the lead 200 is produced in thelead 201. The average intensity of the light reaching VTI and VT2produces a statistical noise voltage across R1 and R1. This noisevoltage is amplified by the amplifier system and finally develops anamplified noise voltage in the cathode circuits of VTS. The noisevoltage is conducted through VT11); rectified. and the A. C. componentis amplified by VT11. The amplified output of VT11 is fed to peak-typerectifier VT12. The rectified output of VT12 is fed to D. C. amplifierVT13 through time circuit R41 and C19 and the conductivity of VT13 ischanged with a time lag of approximately ,5 of a second. Thus.statistical noise from the phctocells produces an average voltage acrossresistance R16, and determines the screen grid bias on tubes VT3, VT4,VT5 and VT5. The nature of the circuit incorporating VT12 and VT13 issuch that, when the statistical noise from the photocells increases. thescreen grid bias on tubes VT! through VTS is reduced to such an extentthat the peak value of the amplined noise which appears at the cathodesof VT! is held essentially constant. The value of this nearly constantnoise level at cathodes of VT! is automatically adjusted to a valuewhich is a little less than is necessary to cause any activity of thetrigger circuits to be described later. Due to the time constant C19 andR4! the system described above does not respond rapidly enough toprevent the amplification of a signal of short duration of the typeutilized with the present invention.

If equal abrupt changes occur in the intensity of the light reachingboth photocells VTI and VT2, these changes balance out in thedifferential amplifier circuit due to the push-pull arrangement thereof.

It can be seen then. that the circuit of Fig. 7. will, by proper choiceof circuit constants. amplify only signals of substantiallyinstantaneous duration and only the difference between the signalsreaching photocells VII and VT2.

Preferred values for the various elements shown in Fig. 'I are asfollows:

Resistors RI and RI .5 megohms watt R2 3 k. V: watt R3 50 1:. V2 watt R4and R5 5 megohms 5% watt R6 50 k. A watt R1 and R8 200 1:. ti watt RIOandRll 3 megchms 5; watt Rl2 50 k. A watt RI: 2 1:. $5 watt Ri4 andRl5200 k. lawatt RIB .5 megohms 1 watt "RI'I 5k.1watt RllandRis 100 1:.%watt R k. watt R2| 1000 ohms watt R22 and R23 l megohm 55 watt R24 andR24 megohm ,9 watt R25 andR25' 1 megohm 5 watt R26 .25 megohm watt R2150 1:. watt R20 25 k. watt R20 and R29 100 k. watt R20 and R20 5000 ohmswatt RSI andRil' .25 megohm R22 .5 megohm R32 l megohm R34 .25 megohm 95watt R81 1000 ohms R20 5 k. pot. WW 4 watt R39 7.5 k. 1 watt R40 andR40".5 megohm $6 watt R4! 2 megohms 55 watt Condensers C' .1 t. paper 400 v.

C2 and C2 200 i. mica Cl .0005 pf. mica C4 .0005 ,mmica C5 .25 i. paper400 v.

C8 and C1 .001 #1. paper 600 v.

Cl .5 pf. paper 400 v.

C! and CW .001 t. paper 600 v.

C" andC|2 .01 pLpapei-GOOv.

Cll .1 p1. paper-400 v.

C" 1.0 f. paper 400 v.

20 Condensers-Continued Cl! and CIG .005 #1. paper 600 v. CI! Smallamount of twisted wire Cl 8 .l uf. paper 400 v. Cl! 1.0 af. paper 400 v.C20 .25 f. paper 400 v. C2i .1 #f. paper 400 v. C22 .001 #1. paper 600v. C23 1.0 af. paper 400 v. C25 .25 1. paper 400 v. C26 .01 pf. paper600 v.

Vacuum tubes VT! and VT2 Farnsworth Special Photocell VT3,VT4, VTS,andVTt 12SJ7 VT'I and VT8 12SH'7 VTS 12SL7 VTI 0 12116 W! I 12SJ7 (TI 212H6 VTI2 128.77

The interpreting circuit used in connection with the differentialamplifier and automatic volume control circuit of Fig. '7 is shown inthe schematic diagram of Fig. 8. In this figure. leads 200. 20! and 202correspond to the leads of the same numbers in Fig. 7. Lead 200 iscoupled to gas tube VTI4 by condenser C21. The first grid of VTI4 isbiased at ground potential by resistors R42 and R42. while the cathodethereof is connected to a +4.5voltage supply through resistors R40 andR41. The second grid of VT is directly returned to the same bias supplyas the cathode. The plate of VTI4 is connected to a l-volt supplythrough resistances R48 and R86, R46 being lay-passed by condenser C29.

Lead MI is coupled to gas tube VTI5 by means of condenser C24. The gridof this tube is biased at ground potential by resistances R42 and R44.The cathode of VTI5 is connected to the +4.5 voltage supply throughresistances R40 and R41. The second grid of Vii is directly returned tothe same bias supply as the cathode. The plate circuit of VTI5 includesresistances R46 and R56, R45 being by-passed by condenser C28.

The cathode of VTI4 is coupled to double diode VTIG by means ofresistance R49 and the cathode of VTIS is coupled to double diode VTIIby means of resistance R49.

Between the cathode of VTI4 and screen grid .Of VT20 there is provided atime circuit containing a resistance R50 and a condenser C30. Betweenthe cathode cf VTI4 and the control grid of VTIB. there is another timecircuit. includin the resistance R52 and condenser C32. The time circuitR50 C20 has a shorter time constant than the time circuit R52 C32. Aresistance R54 is included in the grid circuit of VTll to limit the gridcurrent therein. The tube VTIi is connected across the time circuits topermit the rapid discharge of the condensers C30 and C32 at the end ofthe cycle.

Between the cathode of VTII and screen grid of VT2! there is provided atime circuit containing a resistance RBI and a condenser CM. Between thecathode of VTII and the control grid of VTIS there is another timecircuit, including the resistance R53 and condenser C32. The timecircuit RSI Cll has a shorter time constant than the time circuit R52C22. A resistance R55 is 21 included in the grid circuit of VTIQ tolimit the grid current therein. The tube VTII is connected across thetime circuits to permit the rapid discharge of the condensers C3! andC33 at the end of the cycle.

The cathodes of tubes VTIB, VTIS, VT20 and VT2| are all connectedtogether. Between their common connection and ground there is provided agas diode VT23. The shield grids WM and VTI9 are connected to a +45-voltsupply. The common cathode junction of the tubes VTi3. VTIS, VT20 andVT2I is retumed to plus 45 volts through the load resistor R5! and thedecoupling network R56 and C34.

The control grid of VT20 is coupled to lead 2! through resistance RSIand condenser C38 and is connected to a 40.5-volt bias supply throughresistances RSI and R58. The control grid 0! VTZI similarly is coupledto lead 200 through resistance R50 and condenser C31 and is connected toa 40.5-volt bias supply through resistances R60 and R59.

In the plate circuit or each of tubes VTI3. VTIS, VT20 and VT2l there isa relay. Relay RLi is in the plate circuit of VTI8. When the tube VTiBis conducting the relay RLI closes switch S5, thus connecting terminalU5 with terminal U0 of a utilization circuit which may be the controlsof a guided missile. A condenser C30 is used in parallel with RLI as astorage of energy for maintaining a current flow through RLI for a shorttime after VTI8 has become nonconducting.

Relay BL! is in the plate circuit of tube VTI3. When VTI9 is conducting.Rm closes switch 50. thus connecting terminal U4 with terminal U3. Anenergy storing condenser C35 is supplied for RLZ. In the same manner.the firing of tube VT20 operates through RL3 to close switch S1 andconnect together terminals U1 and U8. The firing of VTZI likewiseconnects terminals U! and U: by means or switch S8 operated by RL4.

A time circuit R64 CM is provided between the common cathode circuit oftubes VTIO through VTZI and the grid circuit of VT24. A resistance R62is provided in the control grid circuit of VT24 to limit grid currenttherein. By-passing the resistance R64 is the double diode VT22, whosepurpose is to permit the rapid discharge or condenser C4l when thecircuit is returned to an inoperative condition. The cathode VT24 isby-passed to ground by means of condenser C42 and is returned to a plus67.5-vo1t supply through resistor R63. The screen grid of VT24 isreturned directly to the plus 67.5-volt supply. The plate load or VT24consists or the parallel combination of R66 and C43. C43 is connectedbetween the plate of VT24 and ground; R60 is connected between the plateand the plus 180 volts. The plate of VT24 joins the common plate circuitsupply for all other gas tubes. namely VTI4, VTi5, VTI3, VHS and VT20and VTZI. Thus, when VT24 conducts, a large voltage is generated acrossR65 which lowers the plate voltages on the tubes mentioned abovesumciently to extinguish any of those tubes which are firing. The effector the condensers C43 and C42 is such that vacuum tube VT24 is alsoextinguished at the same time as the other gas tubes. Condenser C44 isconnected between the l80-volt supply and ground.

Preferred values for the various elements of Fig. 8 are as tollows:

22 Resistors R42 and R42 lmegohm /z watt R43 and R44 .25 megohm V wattR45 and R45 k. V watt R41 25 k. watt R48 and R48 40 k. watt R49 and R4310 k. V. watt R50 and RSI 100 k. V; watt R52 and R53 .5 megohm V wattR54 and R55 100 k. V watt R53 2.5 k. 1 watt R51 5 k. 1 watt R58 and R59lmegohm /2 watt R60 and RSI .25 megohm watt R63 100 1:. V watt R04 2megohms ;& watt Condensers C24 and C21 .001 t. 600 v.

C23 and C23 .00025 af. 600 v. paper C30 and C3 l .01 t. 600 v. paper C32and C33 .02 at. 600 v. paper C34 .25 at. 400 v. paper C35 and C38... .5l. 400 v. paper 031 and C33.. .001 aaf. 600 v. paper C33 and C40 .5 t.400 v. paper C4! .075 t. 600 v. paper C42 .25 of. 400 v. paper C43 .1pl. 600 v. paper C44 1. at. 400 v. paper Vacuum tubes VTI4. VTIS, VTI8,VTI3, VT20, VTZI, and

Nora: Filaments or VTI4 and VTI5 should be connected in series.Filaments or VTIO and VT20 should be connected in series. Filaments ofVTlS and WM should be connected in series.

In the operation or the circuit shown in Fig. 8. it is advisable toconsider first the conditions existing at the beginning of a controlcycle when none or the gas tubes is conducting. At this time, thereexists a bias of minus 4.5 volts between the control grids of VTI4 andVT! 5 and their respective cathodes. The shield grids of tubes VT20 andVTII are at plus 4.5 volts potential, referred to ground, because thereis no charge on the condensers C30 or C3l, respectively. The controlgrids of VT20 and VTZI are at plus 40.5 volts. while the cathodes ofthese tubes are at approximately plus 45 volts. Thus, in each or thetubes VT20 and VT, there is a negative bias of 4.5 volts between thecontrol grids and cathodes and a negative bias of 40.5 volts on theshield grids with respect to the cathodes.

Both the shield grids and cathodes of tubes VTI3 and VTIS are at a plus45 volts potential. while the control grids of these two tubes are atplus 4.5 volts potential. Thus, only the control grids of these twotubes have a negative bias with respect to their cathodes.

when a positive pulse enters the circuit from lead 200, it triggers tubeVTI4. As this tube conducts, the potential of its cathode is raisedconsiderably above plus 45 volts, causing a charge to flow intocondensers C30 and C32, the time constant of the charging circuit of C30being shorter than that of C32. Hence. the potential across C30 risesmore rapidly than that across C33. and the shield grid of tube VT20 israised sufilciently to allow that tube to be triggered by a signal onits control rid. if one occurs in the short interval of time before thecontrol grid of VTil is raised sufficiently to trigger VTI8.Accordingly, either V'IIll or VTIB will fire, depending on whether ornot a positive signal appears on lead "I during the specified interval.When either tube does fire, the cathode potential of all tubes VTI!through VT2I is raised, due to their common cathode resistors R56 andR51, placing a large negative bias on the control grids of thenonconducting tubes with respect to their cathodes. This bias preventsthese tubes from firing as long as the conducting tube continues toconduct.

It should be pointed out that, due to the large common cathoderesistance R" in the circuits of VTI! and VTIS when one of these tubesis conducting, the cathode of the other tube is' raised to such a largepotential with respect to its grid, that a signal tending to fire thattube will be ineffective to permit it to fire. Thus, if VT I4 is firingas described above, and a positive signal appears on lead 20!, thispositive signal cannot fire VTIS.

It should be noted that VT23 is connected between the common cathodejunction of tubes V'Iil through VT2I and ground. When the potential atthis common junction rises due to the conduction of one of tubes VTIBthrough VT2I, VT will be made conducting and will allow a large currentfiow to pass through the particular tube that is firing, therebyenabling the relay in the plate circuit of that tube to close the switchassociated therewith and to allow the condenser. in parallel with therelay, to store up sufficient energy to maintain said relay closed afterits associated tube has become nonconducting.

VTZ3 also has an important voltage regulating function, in that once itstarts to conduct. the voltage drop'thereacross will be constant. Thisvoltage is preferably around 90 volts and therefore, the chargingvoltage for the circuit R64 Cll will remain constant. It is importantthat the charging voltage of the time constant R64 and Cll be heldconstant, since the rate of charge on the condenser CH determines thetime at which the tube VT will fire and therefore, controls the lengthof the cycle. Since. when VT! conducts, it lowers all of the platepotentials of the tubes VT and VTIS, and VTI! through VT! I. it willextinguish any of those tubes which are firing. After VT has conductedfor a short time, it will extinguish itself, due to the action of thecondensers C42 and C43. At this point, the circuit will return to the,condition assumed at the beginning of this'discussion. The circuit isthenready for the next signal.

In summation, it can be seen that the following conditions hold true ofthe circuit shown in Fig. 8. If a single positive pulse enters thecircuit from lead 200, after a short delay VTIB fires. But, if apositive pulse entering the circuit from lead 200 is followed promptlythereafter by a positive pulse from the lead 2M, VTZU fires. Conversely,if a single positive pulse enters the circuit from lead 20 I, after ashort delay, VTI! fires. But, if a positive pulse entering the circuitfrom lead 21H is followed by a positive pulse entering the circuit fromlead 200, VTII fires.

Assuming that VTIB corresponds to an "up" signal, VTI9 to a down"signal, VT20 to a. right" signal. and VTZI to a "left" signal, theoperation of this circuit on signals generated by a transmitter of thetype shown in Fig. 6 can be readily understood. If an up signal isgenerated in Fig. 6, it will consist of a single flash by TVTI of Fig.6. Assume then, that TVTI and VTI of Fig. "I have similar lightpolarizers and light filters in front thereof. A single signal generatedby TVTI will energize VTI and produce a positive pulse in lead 200 ofFig. 7 and Fig. 8. This single positive pulse, as explained, will fireVTil. Assume now, that a. right signal is given in the transmitter ofFig. 6. This consists of a flash by TVTI followed promptly by a fiash byTVTI. The flash of TVTI produces a positive pulse in lead 200 of Figs. 7and 8, and the flash from TVT2 produces a positive pulse in the lead 2thof Figs. 7 and 8. As previously explained, this combination of pulseswill fire tube VTIO. By this same reasoning a down signal will fire VTI!and a left signal will fire VTZ l.

The time constant R64 and Cl! which controls the firing of VT of Fig. 8is closely coordinated with the charging condenser in parallel with thevarious relays of Fig. 8, and also with the timing of the rotation ofthe commutator ill! in Fig. 6. The reason for this is that it is desiredto fire the various tubes VTIB through VTZl, close. the

corresponding relays, and store up enough energy in the condensers tokeep those relays closed for an appreciable time, and then extinguishtubes VTI8 through VTZI so that the circuit may be made ready for thenext signal.

Suppose, for example. it is desired to effect both an up and rightcontrol simultaneously. by closing both switch SI and S2. Looking atFig. 6. when the shorting bar Iii reaches contacts H2 attached to lineE, an up signal is produced because switch Si is closed. This signal istransmitted by TVTI and, as explained previously, fires tube VTit ofFig. 8. This tube closes relay RLI and charges condenser C35. when asufficient charge in C36 has been built up, the time constant RG4 C willtrigger VT. thus extinguishing VTIG. The condenser C35. however, willmaintain the switch S5 closed while the shorting bar iii of thecommutator ill! in Fig. 6

reaches contacts H3 and ill. and. since 82 is closed, a right signal istransmitted, and VT" fires, closing switch S1. Thus. both a right and anup signal can automatically alternately be fed to the utilizationcircuit, and two final relays can be kept closed simultaneously. Whenthe other end of the shorting bar Iii of the commutator H0 reachescontact H2, the operation can be repeated. Thus, by means of thiscircuit, two appropriate switches may be maintained continuously closed,while the signals energizing the circuit are of substantiallyinstantaneous duration.

Since certain changes in carrying out the above process and in theconstructions set forth which embody the invention may be made withoutdeparting from its scope. 'it is intended that all matter contained inthe above description. or shown in the accompanying drawings. shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:

1. In a light sensitive receiver, a first and a second light sensitivedevice each responsive to individualized signals thereto. an amplifierresponsive to the difference in response of said first and second lightsensitive devices to produce a signal whose polarity depends upon whichof said light sensitive devices is producing the stronger response, afirst pair of normally nonconducting grid controlled gas tubes connectedto said amplifier and responsive to the polarity of a signal from saidamplifier to render one or the other of said tubes conducting, and meansresponsive to the conduction of one tube to render the other tubeunresponsive to signals from said amplifier, a short time intervalcharging circuit and a long time interval charging circuit connectedwith each of said first pair of gas tubes, a second pair of normallynon-conducting gas tubes having control grids connected to said longtime interval charging circuits respectively, a third pair of normallynon-conducting gas tubes each having a control grid and a biasing grid,said biasing grids being normally biased to render said tubesnon-responsive to the control grids and connected with said short timeinterval charging circuits respectively to render said third pair oftubes responsive to said control grids when the respective one of saidfirst pair of gas tubes is conducting, means responsive to conduction ofany one oi said second and third pairs of gas tubes to render theremainder of said tubes unresponsive to the control grids thereof, andconnections from the output 01 said amplifier to the control grids ofsaid third pair of gas tubes, whereby a sequence or two signals fromsaid amplifier will render one or the other of said third pair of tubesconducting while a single signal will render one or the other of saidsecond pair of tubes conducting. time delay means to render said first,second, and third pairs of gas tubes non-conducting after apredetermined time interval, and utilization means connected in theoutput circui of saidsecond and third pairs of t bes.

2. In a communication system a first and a second flash tube, a firstand a second light sensitive device, a first pair of light polarizingfilters each having a first predetermined polarization axis associatedwith first fiash tube and said first light sensitive devicerespectively, a second pair light polarizing filters each having asecond polarization axis perpendicular to said first polarization axisassociated with said second flash tube and said second light sensitivedevice respectively, an energization circuit for each of said flashtubes, a motor-operated switch element, a plurality of spaced fixedcontacts co-operating with said motor operated switch element and aplurality of manually operative selector switches interconnecting saidfixed contacts and said energization circuit, whereby said motoroperatedswitch element causes energization of said flash tubes in a selectedsequence and at predetermined intervals, an amplifier circuit responsiveto differences in response between said first and second light sensitivedevices to produce a polarized signal therefrom, integrating meansconnected to said amplifier, and circuit means responsive to saidintegrating means to control the gain of said amplifier, a first pair ofnormally non-conducting gas tubes responsive to the polarity of thesignal from said ampliher to render one or the other of said tubes con-Y 26 ducting and means responsive to the conduction of either tube torender the other of said first pair of gas tubes unresponsive to signalsfrom said amplifier device, a short time constant charging circuit and along time constant charging circuit connected in the output circuit ofeach of said gas tubes, 9. second pair of grid controlled gas tubeshaving their grids connected to said long time constant charging circuitrespectively, a third pair of gas tubes each having a biasing grid and acontrol grid. said biasing grids being normally biased to maintain saidthird pair of tubes unresponsive to the control grids thereof andconnected to said short time constant charging circuits respectively torender said tubes responsive to the control grids. means connecting thecontrol grids of said third pair of gas tubes to the output of saidamplifier, whereby said third pair oi. gas tubes is responsive to subsequent signals therefrom, means associated with said second and thirdpairs of gas tubes responsive to conduction of any one of said tubes torender the remainder of said second and third pairs of tubesunresponsive to signals, time delay means responsive to the conductionof any one of said tubes to render said first, second and third pairs orgas tubes non-conducting. and slowopening relay means connected in theoutput circuits of said second and third pairs of gas tubesrespectively, whereby said relay means reproduce the operation oi saidselector switches.

PAUL H. LEE.

REFERENCES CITED The following references are of record in the file ofthis patent:

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